M. A. Beer and G. W. Hammett

Theory of Fusion Plasmas Workshop, Varenna, Sept. 1998

We investigate the dynamics of small-scale turbulence-driven sheared
**ExB** flows in nonlinear gyrofluid simulations. The
importance of these zonal flows in the regulation of the turbulence
was shown in our early simulations[1,2,3]
and has been widely confirmed. Most
of these flows experience fast collisionless linear damping, but there
is a residual non-Maxwellian component of the flow which is undamped[4]
and scales with r/R. In our original treatment, we
included collisionless damping terms which capture the fast
collisionless damping of the damped components, but which do not
accommodate the linearly undamped components. Here, we modify the
gyrofluid closures to account for Pfirsch-Schluter heat flows. This
modification allows a linearly undamped component of the **ExB
**
flows, and here we begin
to assess its importance in nonlinear simulations. Our
preliminary results indicate very near
marginal stability zero-flux states can exist where the undamped zonal
flows completely damp the turbulence, as shown in Ref. 5.
But away from marginal stability, we find that retaining the
undamped component of the flow has very little effect.

1. W. Dorland, Ph. D. thesis, Princeton University (1993).

2. G. W. Hammett, M. A. Beer, W. Dorland, S. C. Cowley, and S. A. Smith,
Plasma Phys. Controlled Fusion **35**, 973 (1993).

3. M. A. Beer, Ph. D. thesis, Princeton University (1995).

4. M. N. Rosenbluth and F. L. Hinton, Phys. Rev. Lett.
**80**, 724 (1998).

5. A. M. Dimits, D. E. Shumaker, W. M. Nevins, B. I. Cohen, and S. E. Parker, Sherwood Fusion Theory Conference (1998).